There is a certain attitude that you should have as a main tank. If you are unsure of yourself, it will show to the people around you. There are many things that you can do to make yourself stand out as a "Main" Tank type. Being a main tank doesnt always have to mean being THE main tank.
Just because someone else is tanking the boss this fight does not make you any less of a main tank. Off-tank is a role for a given fight. Main tank is a way of thinking and doing things. In order to be a Main Tank, the people around you have to believe in you as the Main Tank.
Main tank does not mean that you are the number one go-to tank in a raiding guild. I have seen a few "main tanks" that were not very good at all.
There are serveral things that you can do to prove yourself as a Main tank.
1) Take Charge: And Im not talking about Warbringer (though as a Warrior tank you better darn well have it). If you are running something, be a leader. Explain fights if need be, mark targets if you arent just AOEing everything down, and dictate the pace of the instance. It doesnt matter if you are in Ulduar or in Sunken Temple (if you are in Ulduar, and not the raid leader, dont go crazy). People will notice that you are well prepared and will enjoy running with you. The less your dps have to think, the easier the run will be.
2) Be confident: Nobody wants to run an instance with a tank who is not confident in their abilities. If you are hesitant, you are much more likely to make mistake. A confident tank can and will adapt to situations that occur outside the normal script of things. Part of being confident is not the ability to perform under pressure and not panic. If you panic, your thought process may well go out the window and the entire encounter will go south. Staying calm and cool in the face of adversity will help you develope as a main tank.
3) Don't be afraid to take the blame: Don't be the guy who hides in the corner after you caused a wipe. Come out and say it. If you know that you caused a wipe, and the people around you know the YOU KNOW what you did, then they are going to trust you even further. If you cause a wipe and deny it, you are just going to piss people off. If you admit your mistake and make the corrections needed, you will be acting as a true Main Tank.
4) Know your stuff: You should know the standard tanking rotation for your class. Your job as a tank is to put out enough threat to keep the dps from getting smashed to bits. It is also your job to stay alive. You should know what cooldowns you have available to you and what situations you should be using them in. If you died in a fight and had defensive cooldowns that were not used, it may well be your fault.
5) Tank your shit: This one is pretty simple, and I believe that it is the Holy Grail of tanking. Thou shalt tank your shit. Mobs should not be running here, there, and everywhere. You have tools to keep mobs on you. Use them. If mobs are running around and killing everyone, it is your fault if you have the ability to stop it. There are occasions when you will want to let that one mob go smash the overzealous Warlock in the face. That is another story. If it is assigned to you, handle it.
Obviously, there are other things that go into being a Main Tank type, but this a good start for any of you aspiring tanks out there. There is so much information out there, that you should not be going into tanking blind. If you are reading this blog, then you obviously are aware of these sources and this probably doesnt apply to you 0_o.